In recent years, in order to respond to the expanded functionality and power savings of electronic equipment such as portable equipment, power supply voltages are switched according to an operating state in some cases. FIG. 5 shows a conventional power supply having the function of switching power supply voltages.
The power supply of FIG. 5 is identical to a conventional example shown in FIG. 2 of (Patent Document 1). Reference numeral 1 denotes an input terminal, reference numeral 2 denotes an output terminal, reference numeral 3 denotes a control terminal, reference numeral 4 denotes a converter unit including a series regulator and a switching regulator, and reference numeral 6 denotes an output detection feedback section.
The converter unit 4 has the input terminal 1 and the output terminal 2. The converter unit 4 transforms power supply input voltage Vin, which has been applied to the input terminal 1, to desired output voltage Vout based on an error signal Ve inputted from the output detection feedback section 6. Then, the converter unit 4 outputs the output voltage Vout from the output terminal 2.
The output detection feedback section 6 comprises a series circuit with resistors 31, 32, and 33 for dividing the output voltage Vout, a switch 34 for short-circuiting/opening both ends of the resistor 31 according to a control signal Vs inputted from the control terminal 3, and an error amplifier 36 which compares a voltage at the node of the resistors 32 and 33 with a reference voltage Vref of a voltage source 35, amplifies the voltage, and outputs the error signal Ve.
The converter unit 4 of (Patent Document 1) is a series regulator comprising a P-channel transistor. However, various circuits including switching regulators are suggested and generalized as the converter unit 4.
As described above, the converter unit 4 is a circuit for transforming the power supply input voltage Vin to the desired power supply output voltage Vout according to the error voltage Ve, and the converter unit 4 operates in such a way that the positive and negative input terminals of the error amplifier 36 are equal in voltage. Therefore, the switch 34 is turned on/off in response to the control signal Vs and a voltage division ratio is changed depending upon whether the resistor 31 is short-circuited or not, so that the output voltage Vout can be switched. Assuming that the resistors 31, 32, and 33 have resistances R31, R32, and R33, respectively, when the switch 34 is turned off, an output voltage Vout1 is expressed by the equation below:Vout1=Vref·(1+R31/R33+R32/R33)When the switch 34 is turned on, an output voltage Vout2 is expressed by the equation below:Vout2=Vref·(1+R32/R33)
In this configuration, however, when the output voltage is changed, the input terminal voltage of the error amplifier 36 considerably changes in a short time. As a result, overshoot or undershoot occurs at the output voltage. In Japanese Patent Laid-Open No. 11-119845, in order to solve this problem, a low-pass filter is inserted to the input of the error amplifier 36 and the low-pass filter is operated for a fixed time period after a voltage division ratio is changed, so that a change of the input terminal voltage to the error amplifier 36 is reduced.
In the conventional power supply, however, when the output voltage is changed, a detected resistance is changed by the switch 34 to change a resistance division ratio. Thus, an input impedance to the error amplifier 36 is changed and the alternating gain of the error amplifier 36 is also changed, which complicates a design for obtaining the control stability of the power supply in response to the switching of the output voltage. Further, when the alternating gain of the error amplifier 36 is reduced by turning off the switch 34, a limitation on overshoot is reduced. The overshoot occurs when the output voltage increases.
Moreover, in Japanese Patent Laid-Open No. 11-119845, in response to overshoot or undershoot occurring on the output voltage, the low-pass filter is inserted to the input of the error amplifier 36 and a change of the input terminal voltage to the error amplifier 36 is reduced, thereby reducing the speed of changing output voltage. That is, even when the switch is operated in response the control signal Vs, output voltage is switched with a time lag.
An object of the present invention is to provide a power supply whereby when output voltage is switched, the output voltage can be quickly switched without causing switching time delay while overshoot and undershoot is reduced, and the control stability of the power supply can be easily obtained in response to a change of the output voltage.